In order to realize high integration and ultramicronization of a semiconductor device developing from a half pitch of 65 nm into 45 nm and further 32 nm, a high-NA technique for increasing the numeric aperture of a projector lens, an immersion exposure technique for exposing while making a high-refraction medium intervene between a projector lens and an exposure target, and an off axis illumination-mounted exposure technique have been put to practical use as a high-resolution technique in an exposure device in photolithography.
Phase shifting masks such as a Levenson (also referred to as Shibuya-Levenson) phase shifting mask for improving resolution by a phase shifting effect utilizing optical interference, a halftone phase shifting mask (simply referred to as a halftone mask hereinafter) constituted from a part for transmitting light and a part for semi-transmitting light, and a chromeless phase shifting mask including no light shielding layers such as chrome have been used as measures to improve resolution in a photomask (also referred to as a mask hereinafter) used for photolithography, together with micronization and high precision of a conventional binary mask constituted from a part for transmitting light and a part for shielding light.
In a photolithographic technique, the minimum dimension (resolution) transferable by a projection exposure device is in proportion to a wavelength of light used for exposure and is in inverse proportion to the numeric aperture (NA) of a lens in a projection optical system, so that shorter wavelength of exposing light and higher NA of a projection optical system have been progressing in accordance with a request for micronization of a semiconductor device; however, it has become a limit to make only shorter wavelength and higher NA satisfy this request.
Thus, a super-resolution technique for intending micronization by decreasing a value of process constant k1 (k1=resolution line width×␣numeric aperture of projection optical system/wavelength of exposing light) has been proposed for improving resolution in recent years. Methods called a method of optimizing a mask pattern by providing an assist pattern and a line width offset for the mask pattern in accordance with the properties of an exposure optical system, or a method by an off axis illumination (also referred to as an oblique-incidence illumination method) are offered as such a super-resolution technique. An annular illumination (also referred to as Annular) using a pupil filter, a dipolar illumination using a dipolar (also referred to as Dipole) pupil filter, and a quadrupole illumination using a quadrupole (also referred to as Cquad) pupil filter are ordinarily used for projection exposure by an off axis illumination.
The method using an assist pattern is a lithographic method using a photomask having the effect of improving resolution and focal depth of a main pattern by disposing a pattern (referred to as an assist pattern hereinafter), which is a resolution limit or less of a projection optical system and not transferred on a wafer, nearby a pattern (referred to as a main pattern hereinafter), which is transferred on a wafer (for example, refer to Patent Literature 1). The assist pattern is also called SRAF (Sub Resolution Assist Feature) (the assist pattern is also referred to as SRAF hereinafter in the present invention).
However, in accordance with micronization of a semiconductor device pattern, a photomask having an assist pattern has had difficulty in producing the mask. First, as described above, the difficulty is conceived to be such that the assist pattern itself needs not to be imaged on a wafer and needs to be minuter in dimension than a main pattern. As a result, in accordance with micronization of the main pattern dimension, the line width dimension of the assist pattern to be requested is micronized from several hundreds nm to such a minuter dimension as to be approaching a limit in producing. For example, in the case of forming a semiconductor device of a 65-nm line width on a wafer, the line width dimension of the main pattern on the mask (a reticle with an ordinary tetraploid pattern) is formed into approximately 200 nm to 400 nm in addition to optical proximity correction (OPC), while the line width dimension of the assist pattern becomes 120 nm or less and the mask production becomes extremely difficult. As described above, the dimension of the assist pattern is a great problem in producing the mask on the exposure conditions of transferring a pattern with a half pitch of 65 nm or less.
In addition, with regard to transfer properties of the mask on which a pattern with a half pitch of 65 nm or less is transferred, as described later, a halftone mask allows more favorable transferred image than a binary mask so frequently that it is greatly desired that the mask having the assist pattern is constituted into a halftone mask and a halftone mask having the assist pattern is also proposed (for example, refer to Patent Literature 2, Patent Literature 3 and Non Patent Literature 1). However, a halftone mask ordinarily has a minus bias in the mask pattern dimension by reason of transfer properties, so that it is requested that the dimension of the assist pattern formed from a semi-transparent film as a halftone mask is smaller than the dimension of the assist pattern of a binary mask formed from only a light shielding film. In a generation from 45 nm to 32 nm of a half pitch of a semiconductor device, the assist pattern dimension of 60 nm or less in mask line width has been requested, depending on the design and exposure conditions of a semiconductor.
Also, in accordance with micronization of the assist pattern, in a mask production process such as washing, or in the case of rewashing a mask which became dirty during using in an exposure device, with regard to a halftone mask including a conventional assist pattern, an aspect ratio (pattern height/pattern width) of the assist pattern approaches 1 and the problem is to cause phenomena such that part of the assist pattern is chipped, the assist pattern is peeled off a substrate surface, and the assist pattern falls in the line width direction.
A photomask, in which a retardation of 180° is generated between the light transmitting through a semi-transparent pattern and the light transmitting through a transparent region of a transparent substrate, a predetermined retardation within the scope of 50° or less is generated between the light transmitting through a semi-transparent assist pattern and the light transmitting through a transparent region of a transparent substrate, and focus properties of the semi-transparent pattern are flattened, is proposed in Patent Literature 2 as correspondence to micronization of the assist pattern by a halftone mask. FIGS. 24A and 24B are a plan view (FIG. 24A) and a longitudinal cross-sectional view (FIG. 24B) of a photomask described in Patent Literature 2. The photomask according to Patent Literature 2 allows an assist pattern provided nearby a line pattern as a main pattern to be also formed into the same dimension as the main pattern.
The halftone mask having the assist pattern described in Patent Literature 2, as shown in FIGS. 24A and 24B, is a mask such that a semi-transparent pattern as a main pattern 1 is a line pattern with a line width of 0.3 μm on a wafer and a semi-transparent assist pattern 2 is a line pattern with the same line width as the main pattern 1 on the right and left thereof. In this mask, the main pattern 1 has a two-layer constitution such that a transparent film 304 is further formed on a semi-transparent film 302, a retardation of 180° is generated between the light transmitting through the semi-transparent main pattern 1 formed from the two-layer film and the light transmitting through a transparent region of a transparent substrate 301, on the other hand, a predetermined retardation within the scope of 50° or less is generated between the light transmitting through the semi-transparent assist pattern 2 and the light transmitting through a transparent region of the transparent substrate 301, and focus properties of the semi-transparent pattern are flattened.